In processes used in blood cleansing therapy such as hemodialysis, hemofiltration and hemodiafiltration, blood from a patient is conveyed through an extracorporeal blood circuit in which there is a dialyser or filter, which is divided by a semi-permeable membrane into a blood chamber and a dialysis-fluid chamber or filtrate chamber. In hemodiafiltration, both hemodialysis and hemofiltration are performed. The present invention relates to all processes used in blood cleansing therapy in which blood flows through the blood chamber of a dialyser and dialysis fluids flows through the dialysis-fluid chamber.
There are various known physical and/or chemical metrics by means of which the performance of the dialyser and/or the effectiveness of a dialysis treatment can be specified. One known metric for the performance of a dialyser is the clearance K. The clearance K of a substance is that proportion of the total flow through the dialyser which is totally cleared of the substance concerned. For the effectiveness of a dialysis treatment, what is of crucial significance is what is termed the dialysis dose (Kt)/V, which is defined as the quotient of the product of clearance K for urea multiplied by effective treatment time t, divided by the volume V of the patient's body through which urea is distributed.
U.S. Pat. No. 5,100,554 describes a method of determining clearance in which dialyser electrolyte transfer is measured at each of two different dialysate input concentrations. It is known from U.S. Pat. No. 5,100,554 that the effectiveness of dialysis treatment is dependent on the blood flow and the dialysis-fluid flow.
DE 695 31 137 T2 (WO 95/32010) describes a method and arrangement for optimising the effectiveness of a dialysis treatment, in which a metric characteristic of the effectiveness of the dialysis treatment is measured during the treatment and a parameter of the dialysis treatment is determined on the basis of the metric to allow optimum effectiveness to be obtained for the dialysis treatment. It can be deduced in detail from the publication that a processor increases the selected parameters, for example the blood flow rate, in a stepwise manner in defined increments, a urea sensor continuously measuring a characteristic variable, in that a sample of the draining-off dialysate is taken. By comparing the currently measured concentration with the preceding concentration, it is ascertained whether the current concentration is smaller or greater than the preceding concentration. If the current concentration is smaller than the preceding concentration, it is concluded that the preceding concentration represents the optimum value. As a result, the maximum value of the concentration is intended to be determined.
In the past, known pieces of dialysis apparatus have been operated with a constant flow of dialysis fluid which could not be altered by the user. More recent pieces of apparatus on the other hand allow different dialysis-fluid flow rates, such for example as 300, 500 and 800 ml/min, to be set manually. To achieve a high clearance, quite high dialysis-fluid flows at quite high blood flows are required.
When a given dialysis-fluid flow is being set, although high clearance can be obtained with a high dialysis-fluid flow, the costs of supplying fresh dialysis fluid and of disposing used dialysis fluid go up. What is therefore desired in practice is relatively high clearance for a relatively low consumption of dialysis fluid.
It is known that, if the widely employed dialysers are used at a ratio of blood flow to dialysis-fluid flow of 1:2, only a slight reduction in clearance occurs in comparison with a non-variable dialysis-fluid flow of 500 ml/min (J. E. Siegdell, B. Tersteegen, Artificial Organs, 10(3), pages 219-225, 1986).
U.S. Pat. No. 5,092,836 therefore proposes controlling dialysis-fluid flow as a function of blood flow in accordance with preset criteria. In particular, what is proposed is for a dialysis-fluid flow to be set that is obtained by multiplying the blood flow by a constant factor. As well as a linear relationship between blood flow and dialysis-fluid flow, what is also proposed is a numerical data matrix, which, for each blood flow of a given dialyser, gives that dialysis-fluid flow at which a given percentage is achieved of the maximum clearance that would have to exist if the dialysis-fluid flow were assumed to be infinitely high. In practice, the percentage may be, for example, 95%.